Trouble with Optional Lab - Expressions with $project

[ankit1738 (Ankit Kumar)]

For the last optional lab…
Instead of using $gt in project and then checking true or false in $match, I directly used $size with $setIntersection and stored it in a field say count in $project like so

and then in next $match I used $gt to check if count is greater than 0 like so

But this approach gives a different answer and I don’t understand why ?

[007_jb (O Brown)]

What troubleshooting have you done? Have you checked to see the values of count?

[ankit1738 (Ankit Kumar)]

The value of count comes out to be wrong with the approach I’m taking

[007_jb (O Brown)]

@007_jb

Have you checked to see the values of count ?

You called the field count, so I meant whether you’ve checked the values of that field. Perhaps there are some nulls.

Your approach gives me the correct answer.

PS: It’s not a good idea to show one of the lab choices.

[ankit1738 (Ankit Kumar)]

Hey I think I found the issue.

See, In the answer(original) you have used two projection, right. I thought its just because a single project would get too crowded. From the original solution I moved the second project, where we are checking if size of $setIntersection is greater than 0, to the first projection and I got the same wrong answer as what I was getting from my solution.

Now in my approach I was counting size of $setIntersection in the same $project, So as soon as I created a new $project stage and moved the counting part to there, I got the correct answer.

But now my question is why do we need two projection or why in a single projection the answer is coming out wrong?

[007_jb (O Brown)]

@ankit1738

Now in my approach I was counting size of $setIntersection in the same $project

You didn’t mention this, however, this will also not affect the answer.

@ankit1738

I thought its just because a single project would get too crowded.

You were absolutely right to think that… there are two $project stages for improved readability.
Merging the two project stages into one project stage and moving $gt from the project stage to a match stage also produces the same result. This is similar to what I did when I took this course.

PS: You could have edited your post rather than delete it. Anyone reading my post #3 (i.e. #3 after your post is deleted) wouldn’t understand what post I was referring to

[Sonali_Mamgain]

Hi @ankit1738,

Please check your discourse inbox and let me know if you have any questions.

Thanks,
Sonali